The present invention relates to optical amplifiers in communications networks. The present invention relates more particularly to a low-cost, compact, high performance rare earth-doped planar waveguide amplifier for use in metropolitan area networks, and a method for fabricating the same.
Over a decade ago, optical amplifiers were developed to replace electronic repeaters in the telecommunications network due to the high cost of building and operating electronic repeaters, as well as the introduction of bottlenecks caused by electronic regeneration. Optical amplifiers enable xe2x80x9coptical transparency,xe2x80x9d allowing the data stream to travel thousands of miles without encountering the need for electrical regeneration. In particular, long-haul transmission designers started to embrace communications at a wavelength of 1.55 xcexcm using erbium (Er) doped silica glass fiber amplifiers (EDFAs), leading to the massive build-up of the present day communications network infrastructure. The substantial gain bandwidth of the EDFA, together with the use of wavelength division multiplexing (WDM) to combine and separate signals, has enabled transmission of 160 individual wavelengths in the fiber, with the bit rate for each wavelength channel usually 2.5 Gb/s, and continuing to increase (to 10 and even 40 Gb/s).
Since the introduction of optical amplifiers, the telecommunications infrastructure and market have grown explosively due to the burgeoning demand for communications and information. In particular, the increased demand has led to the growth in infrastructure of xe2x80x9cmetropolitan area networksxe2x80x9d (MANs or simply metro) where broad bandwidth is brought closer to and made directly accessible by consumers. FIG. 1 illustrates the core relationship of MANs 11 with access networks, such as broadband cable TV. While the direct optical amplification with EDFAs (13 in FIG. 1) has been successful in meeting the need for raw bandwidth in the long-haul portion (12 in FIG. 1) of the network, it is considered too expensive for widespread use in MANs, and very bulky with fiber lengths of about 40 m.
Thus, in order to address the increased need for raw bandwidth for metro and consumer access to broadband, metro networks require a compact, reliable, high-performance 1.55 xcexcm optical amplifier capable of low cost commercial mass production. Such an optical amplifier would enable new functionality (i.e. cost, integration, and/or alternative hosts) for the metro and access networks. It is expected that the transmission rates per wavelength will continue to increase from 2.5 Gb/s to 40 Gb/s. Without low cost amplifiers widely implemented throughout the metro and access networks, the higher transmission rates will reduce the reach of the transmitters, limiting the ability to upgrade the system.
One aspect of the present invention includes a method of fabricating a monolithic cladding-pumped optical waveguide amplifier, comprising the steps of: fabricating a multi-layer planar construction having n rare earth-doped core layer(s) and n+1 cladding layers in alternating layered arrangement, and top and bottom surfaces revealing a cross-section of the layers, wherein the core layer(s) has a higher refractive index than the cladding layers; and diffusing ions through at least one of the top and bottom surfaces into the core layer(s) and the cladding layers to form ion-diffused regions thereof having respectively increased refractive indices, the ion-diffused regions forming an ion-diffused block laterally situated between non-diffused regions of a pair of partially-diffused cladding layers. In this manner, the ion-diffused region(s) of the core layer(s) forms a signal waveguide(s) for carrying signals therethrough, and the ion-diffused regions of the cladding slabs together form a cladding-pump waveguide for optically pumping the signals carried through the signal waveguide(s).
Another aspect of the present invention includes a method of fabricating a monolithic cladding-pumped optical waveguide amplifier, comprising the steps of: bonding n core slab(s) with n+1 cladding slabs in alternating layered arrangement to produce a multi-layer planar construction, wherein the core slab(s) has a refractive index greater than the cladding slabs; slicing the multi-layer planar construction into at least two multi-layer planar units each having top and bottom surfaces revealing a cross-section of the slab bonding; on each multi-layer planar unit, producing by metal deposition and photolithography an ion-diffusable metallic stripe on one of the top and bottom surfaces and over the core slab(s) and the cladding slabs; and diffusing ions from the metallic stripe into the core slab(s) and the cladding slabs to form ion-diffused regions of the core slab(s) and the cladding slabs having respectively increased refractive indices, the ion-diffused regions forming an ion-diffused block laterally situated between non-diffused regions of a pair of partially-diffused cladding slabs. In this manner, the ion-diffused region of the core slab forms a signal waveguide for carrying signals therethrough, and the ion-diffused regions of the cladding slabs together form a cladding-pump waveguide for optically pumping the signals carried through the signal waveguide.
Another aspect of the present invention is a method of fabricating a monolithic cladding-pumped optical waveguide amplifier from a multi-layer planar substrate, said multi-layer planar substrate having n rare earth-doped core layer(s) and n+1 cladding layers in alternating layer arrangement, and top and bottom surfaces revealing a cross-section of the layers, wherein the core layer(s) has a higher refractive index than the cladding layers, said method comprising the steps of: diffusing ions through at least one of the top and bottom surfaces into the core layer(s) and the cladding layers to form ion-diffused regions thereof having respectively increased refractive indices, the ion-diffused regions forming an ion-diffused block laterally situated between non-diffused regions of a pair of partially-diffused cladding layers, wherein the ion-diffused region(s) of the core layer(s) forms a signal waveguide(s) for carrying signals therethrough, and the ion-diffused regions of the cladding slabs together form a cladding-pump waveguide for optically pumping the signals carried through the signal waveguide(s).
Another aspect of the present invention includes a monolithic cladding-pumped optical waveguide amplifier fabricated according to one of the methods described above.
Some of the advantages of the monolithic cladding-pumped optical waveguide amplifier and method of the present inventions include the lowering of manufacturing costs due to the incorporation of standard lithographic techniques that naturally extend to commercial mass production. The compact slab architecture also enables the use of low cost high-power broad stripe diode pumps which can further lower costs. The waveguide amplifier is suitable for use in both single and multimode high-power waveguide amplifiers. Moreover, the compact cladding-pumped slab architecture of the waveguide amplifier offers increased functionality and performance to low and high-power waveguide lasers involving different hosts and laser ions.